The present invention relates to a gas flowmeter and a manufacturing method thereof and, more particularly, it relates to a thermal gas flowmeter in which a heat-generating resistor or heat element is made up of a silicon (Si) semiconductor thin film, as well as a manufacturing method thereof.
Heretofore, thermal gas flowmeters have been predominant as flowmeters for an amount of intake air provided in an electronic control fuel injection system of an internal combustion engine such as for automobiles, or a flowmeter for various kinds of gases used for the production of semiconductors and for hydrogen/oxygen for use in fuel cells since the thermal gas flowmeter can detect the amount of the mass gas directly. Among them, a gas flowmeter manufactured by semiconductor micro-machining technique has been noted since it can reduce the cost and can be driven at a low power.
As the gas flowmeters using existent semiconductor substrates, those using polycrystalline silicon (polysilicon) instead of platinum used so far have been known in view of the heat resistance and the material cost as the heat element as describe, for example, in Japanese Patent No. 2880651.
However, where polycrystalline silicon (polysilicon) is used as the heat element as described in this patent No. 2880651, there has recently been found a problem of secular change in which the resistance value of the heat element changes with lapse of time.